The most widely used Li-ion batteries are LiCoO2 based cells, which possess good energy density and life cycles. However, cobalt based cathodes are too expensive to be used for large-scale systems such as electric vehicles and renewable energy harvesting systems. Higher energy density, better reliability, and improved safety are also necessary for widespread use of Li-ion batteries. For instance, a single charge of an electric vehicle is unable to sustain current standard driving distance per charge (˜500 km). Lithium-sulfur (Li—S) batteries have been investigated as a possible solution due to high energy density and inexpensive raw materials.
Sulfur undergoes the following overall redox reaction: S8+16 Li++16 e−↔8 Li2S. This reaction yields theoretical capacities of 1672 mAh/g with an average redox potential of ˜2.2 V (vs. Li/Li+). With a theoretical capacity of 3860 mAh/g for the Li metal anode, the theoretical energy density of Li—S batteries can be as high as 2567 Wh/kg. The performance of Li—S batteries is outstanding, compared to theoretical values of popular LiCoO2-graphite based batteries (584 Wh/kg and 376 Wh/kg when LiCoO2 capacity is considered as 274 mAh/g and 140 mAh/g, respectively). Considering ˜ 1/300 of sulfur price compared to cobalt, Li—S batteries are strong candidates as the next generation energy storage devices provided that the following current major drawbacks are eliminated or alleviated.
Significant reduction in actual capacity is partly caused by electrically insulating sulfur due to poor charge transfer, but this has been greatly alleviated by adding electrically conducting carbon structures to sulfur. On the contrary, the Li metal anode in Li—S batteries is unsafe, unreliable, and expensive. Li—S batteries also have relatively fast capacity fading during cycling due to polysulfide shuttle mechanisms, making the lifetime of Li—S batteries shorter than the demand of current energy storage devices.
During the lithiation of sulfur in a cathode, a series of polysulfides (Li2Sx, x=3˜8) are produced as intermediates. They can easily dissolve in organic solvents and thereby diffuse to the lithium metal (anode) side, where they are reduced to solid precipitates such as Li2S and Li2S2. The repeated shuttle process during the cycling of the cell considerably reduces the active mass in the cathode, leading to low columbic efficiency and fast capacity fading due to the polysulfide shuttle phenomenon.
To overcome these issues, intensive research has been focused on trapping the solid elemental sulfur inside various carbonaceous nanostructures (e.g. mesoporous carbon, graphene, graphene oxides, and carbon nanotubes) by impregnating the molten sulfur into inner pores. The polysulfide shuttle was slowed since the pore reserved a portion of dissolved polysulfides. Nevertheless, the large quantity of inactive carbonaceous materials significantly reduces the weight percentage of sulfur in the electrode film (typically 30˜50 wt. % considering the polymer binder and conductive additive), and thus the energy density of the battery cell.
In place of starting with solid sulfur, an approach is to use polysulfide-containing liquid catholyte as an active material rather than avoiding the high solubility of polysulfides in the electrolyte. Compared to the sluggish reaction of insulating solid phases, liquid catholyte can alleviate the aggregation of irreversible S or Li2S, and achieve a higher utilization of active materials. Several recent studies have tried to add the polysulfide-containing electrolyte into the Li—S battery with either a sulfur-containing or a sulfur-free cathode. For the former configuration, polysulfides function as both the shuttle inhibitor and backup active materials For the latter, the electrode with light-weight carbonaceous materials (e.g. Ketjen black, Super P, and carbon nanotube (CNT) paper) is utilized to provide sites for redox reactions and paths for charge transfer.
Additionally, it has been noted that the Li metal in Li—S batteries causes a safety hazard and short life time due to the formation of lithium dendrite and an internal short-circuit, which may result in thermal runaway. Even after decades of research efforts, this problem has not been resolved sufficiently to see commercially available rechargeable batteries with a Li metal anode.
There is therefore a need for an improved lithium anode that can be used to realize the complete benefits of a lithium battery. It is an objective of the present invention to use lithiated Si instead of Li metal. The high theoretical capacity (4200 mAh/g) of silicon as anode makes it ideal to couple the high-capacity sulfur cathode.
There is also a need to achieve a long cycling life from Li—S batteries. This has been accomplished in novel semi-liquid Li—S batteries with highly porous CNT sponges as the “super-reservoir” for the liquid polysulfide catholyte. The Li—S rechargeable battery can be used in multiple applications including, without limitation, in electric vehicles.